The present invention relates to a gas expelling equipment for use in a viscous-material treating apparatus such as a kneader, extruder or the like.
In such an apparatus as an extruder of molten plastic to be molded as a plastic mold product, it is inevitably necessary to expell from the material the gas included in or generated from the same to keep the product free from vacuoles. For example, in a conventional rotary screw type extruder with this problem taken into consideration, the screw is designed to have the pitch made unequally spaced with the coarsely spaced portion localized at the practically central portion of the whole length of the screw, while the barrel or cylinder incasing the screw is provided with a gas exhaust pipe (leading to a exhaust pumping system) at the position corresponding to the above coarsely pitched portion of the screw. With the extruder thus constituted, an external pumping system can exhaust only the gas liberated into a broadened space formed at the coarsely pitched portion of the screw without swallowing up the viscous material being transferred toward the outlet of the extruder. However, an extruder of this type has a decisive disadvantage that the extruder is made uncessarily long. In another type of extruder, instead of providing the extruder screw with a coarsely pitched portion to form there a gas stagnating space, there is equipped, near the gas exhaust pipe, a viscous material restoring device to prevent the viscous material being swallowed up through the gas exhaust pipe. The device is, for example, made up of a pair of screws roughly engaged with each other, and mouted on the extruder barrel or cylinder with the screw axes kept vertical to the axis of the extruder screw. The rotation of the paired screws pushes back the viscous material having a tendency to be drawn out by an external gas exhaust sytem through the gas exhaust pipe. In the case of this extruder it is a disadvantage that the viscous-material restoring device, which is relatively tall, protrudes vertically from the extruder body. Another disadvantage is the device has a complex constitution.